1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
In conventional lithography apparatus using masks, stringent requirements are imposed on the global flatness of the mask to prevent telecentricity errors at the substrate. The relatively large demagnification M (e.g., a reduction by a factor of, for example, about 200-400) used in maskless lithography apparatus (i.e., lithography apparatus using a patterning array) exacerbates the problem. The global unflatness U (i.e., unflatness over a period of about 10 or more mirrors) translates into a telecentricity error TE at wafer level according to the formula:TE=2*M*U  (1)
Accordingly, a global unflatness of 40 μrad, typical for a patterning array, yields a telecentricity error of 16-32 mrad compared to a normal specification of 10 mrad total.
Furthermore, as maskless lithography capabilities increase, the tolerance for error in an exposure decreases. One of the errors that can occur in an exposure are optical aberrations in the exposure optics, which are typically caused by manufacturing and assembly tolerance in the exposure optics. Aberrations can result in total focus deviation, among other exposure errors. Total focus deviation results when a focal length of the exposure optics is not identical in all areas of the exposure beam. In this case, a flat patterning array assembly does not focus onto a flat plane at the image or wafer plane. When the optics are changed to correct for the error, movement of the optics components requires great precision. Otherwise, difficulties arise that can introduce additional error into the system.
Therefore, what is needed is a system and method for reducing aberrations without introducing additional error into the maskless system. Also, what is needed is an arrangement for mounting one or more patterning arrays such that unflatness of the patterning arrays can be reduced.